A momentum has been increased for applying carbon nanotube (hereinafter also referred to as CNT) as a constituent material for Micro Electro Mechanical Systems (MEMS) devices or electronic devices in the field of nanotechnology. For obtaining such devices, it has been demanded for a CNT film structure in which a CNT layer including a film-like CNT aggregate comprising a plurality of CNTs (hereinafter also referred to as a CNT film) is deposited on the surface of a substrate.
In the present specification, “carbon nanotube (CNT) film structure” means a structure in which a CNT layer is disposed on a substrate and denotes a structure used as a wafer, for example, of electronic devices or MEMS devices, and it includes not only a substrate having planar surface but also a substrate having an uneven surface and a substrate of a three-dimensional structure in which the surface of a substrate having a protruded structural portion is covered with a CNT film. Further, “CNT aggregate” in the present specification means a structure in which a plurality of CNTs (for example, at a number density of 5×1011 CNTs/cm2 or more) are aggregated in a layered or a bundled state. Further, “CNT film” referred to in the present specification means a plurality of CNTs aggregated in a thin film or thin-plate shape and includes those of sheet, foil, ribbon shapes.
As techniques for producing such a CNT film structure, there have been known a technique of forming a step difference at the surface of a substrate and growing CNTs from the side wall of the step difference in parallel with the surface of the substrate, that is, a technique of producing a film structure having a CNT layer aligned in parallel with the surface of the substrate by using a chemical vapor deposition method (hereinafter also referred to as a CVD method) (refer to JP-A No. 2003-081622), and a technique of coating a liquid suspension of CNTs on a substrate by a spin coating method thereby depositing a non-woven fabric comprising CNT to the surface of the substrate, that is, a technique of producing a film structure with planar surface having a CNT layer (refer to JP-T No. 2005-524000).
However, in the CNT film structure described in JP-A No. 2003-081622, since a stepped substrate is used, it is substantially impossible to planarize the surface of the CNT layer. In addition, the step difference is intended for forming the CNT layer and this is not necessary as a CNT film structure. Since such a step difference rather hinders the laying of wirings, it involves a problem of making the production of an electronic device extremely difficult. Further, since it is necessary that a substrate used in the process for forming the CNT layer (substrate for synthesizing CNT) is identical with a substrate as a film structure (wafer substrate), it is difficult to form the CNT layer at an optional position of an optional substrate and, in addition, only the substrate which withstands a heat treatment during CNT synthesis can be used for the CNT film structure. Further, the CNT layer formed as described above generally has low density (0.03 g/cm3 or less) and is in a fluffy state. Then, since a liquid agent such as a resist impregnates into a gap between CNTs in a state as formed, shaping processing by well-known patterning technique or etching technique is extremely difficult.
On the other hand, in the CNT film structure described in JP-T No. 2005-524000, a liquid suspension of CNTs has to be coated over and over for obtaining a CNT layer of a desired thickness and this tends to complicate the production steps. Further, while a CNT layer in which a plurality of CNTs are aligned in one identical direction can be provided with a property which is different between the alignment direction of CNTs and a direction perpendicular thereto, that is, anisotropy with regard to electric property (for example, conductivity), optical property (for example, transmittance), or mechanical characteristics (for example, bending property), it is difficult in the CNT film structure described in JP-T No. 2005-524000 to align a plurality of CNTs in one identical direction (providing anisotropy) in view of the production process thereof.
JP-A No. 2006-228818 proposes a technique of synthesizing a plurality of vertically aligned CNTs on a substrate and then turning down them thereby aligning the CNTs in parallel with the substrate. However, as it is apparent that JP-A No. 2006-228818 intends to prevent bundling of CNTs (column 0048), no technical idea of using a plurality of CNTs as an aggregate is not recognized therein. Further also in this case, shaping processing by well-known pattering technique and etching technique is substantially impossible.
JP-A No. 2007-182352 proposes a technique of increasing the density of a CNT aggregate aligned in a predetermined direction thereby enhancing the rigidity thereof. This can solve the problem of the existent CNT aggregate that the density is low and the strength is poor. However, this JP-A No. 2007-182352 does not suggest formation of a CNT layer in which a plurality of CNTs are aligned continuously along a film plane.
JP-A No. 2007-182352 also considers a method of exposing a CNT aggregate to a liquid and then applying a pressure in a direction oblique to the alignment direction by way of a planar plate or the like, thereby turning down the CNTs. However, in this case, since the CNT aggregate is adhered to the plate or the CNT aggregate, undergoes damage or causes warp not being durable to the pressure, it is difficult to obtain a CNT film structure having a CNT layer of high density and uniform thickness. Further, since the thickness of the film is limited to 3 μm to 20 μm and, in addition, since CNTs are turned down, it is difficult to obtain a thin film by using CNTs of large size in the height. With the reasons described above, the structure described in JP-A No. 2007-182352 is not suitable to MEMS application use that requires a highly integrated arrangement.
Further, referring to the alignment, the CNT film structure described in JP-A No. 2007-182352 is formed by obliquely turning down, under pressure, the CNT aggregate aligned vertically from the substrate and the direction of the CNTs on the substrate is forcedly changed by an external force before and after the pressure-turn down treatment. That is, in the technique described in JP-A No. 2007-182352, the way of applying the pressure gives a significant effect on the alignment and it is difficult to obtain a CNT film structure in which a plurality of CNTs are aligned at a high level.
Even when the CNT aggregate vertically aligned above the substrate (state in FIG. 1(a)) is exposed to a liquid, it forms an Island shape as shown in FIGS. 1(b), 1(c), 1(d) (refer to Nirupama Chakrapani, et al., Capillarity-driven assembly of two dimensional cellular carbon nanotube foams. Proceedings of the National Academy of Sciences, 2004.3.23, vol. 101, p. 4009-4012), and a CNT layer of a uniform thickness in which the alignment direction of CNTs are in parallel with the surface of a substrate and directed in one identical direction cannot be formed.